This invention relates to the measurement of the magnetic fields produced by a living organism and, more particularly, to a configuration for a biomagnetometer that measures such magnetic fields.
Living subjects produce biomagnetic fields as a result of bioelectric current flows in their bodies. The bioelectric current flows are produced in the brain, the heart, and the nervous system. The bioelectric current is constrained to flow within the subject's body, but the resulting biomagnetic field extends outside the body.
A biomagnetometer is an instrument that measures the biomagnetic fields that reach outside of the subject's body. The biomagnetometer can therefore measure the result of the internal electrical functioning of the body in an external, noninvasive fashion. The measured biomagnetic fields are used to infer the nature of the bioelectric current flows that produced them, which in turn are used to understand the functioning of the body in normal and abnormal circumstances.
The biomagnetometer has a biomagnetic field sensor which includes a biomagnetic field pickup coil positioned external to the body of the subject. A small electrical current flows in the pickup coil responsive to a biomagnetic field produced by the subject. The electrical current of the pickup coil is detected by a sensitive detector, preferably a Superconducting Quantum Interference Device, also known as a "SQUID". The pickup coil and the SQUID operate in the superconducting state, and are contained within a cryogenically cooled dewar during operation. Other electronics amplifies and filters the SQUID output signal, producing an output signal that is further analyzed to understand the electrical patterns of the body.
It is important to place the biomagnetic field pickup coil as closely as possible to the surface of the body of the subject, because the magnitudes of the biomagnetic fields are small to begin with, and decay rapidly with increasing distance from the subject. One of the ongoing trends in biomagnetometry has been to increase the number and spatial coverage of pickup coils around the subject, because more information can be gained by analyzing a large spatial sample of the biomagnetic field than by analyzing the output of a single pickup coil. Thus, for example, the earliest commercially available biomagnetometers had a single pickup coil, later biomagnetometers had 7 or 14 pickup coils, and current commercial biomagnetometers have 37 or more pickup coils.
The array of pickup coils is placed in a dewar that is curved to fit over the head or other portion of the body of the subject. Conventional practice has been to place the subject in a lying position with the dewar over the head of the subject. The lower end of the dewar is shaped to generally conform to the upper surface of the head.
Larger biomagnetometer arrays have been proposed arranged in the shape of a rigid helmet to cover a larger portion of the head. This approach is limited by the natural variation in head size and shape, so that a single helmet will fit only a small proportion of the population.
Recently, a biomagnetometer dewar configuration has been suggested for use with a subject in a reclining position in this "inverted" dewar design, the cryogenic reservoir is below or to the side of the subject, and the pickup coils and their enclosing dewar are below the head of the subject. This repositioning of the dewar presents some difficult problems for dewar design. In this prior proposed design, the pickup coils are cooled by a flow of low-temperature helium gas evolved from a liquid helium reservoir. The flow of helium gas is conveyed to the pickup coils through tubes.
This inverted dewar design has promise, but configurations proposed to date can be thermally inefficient and difficult to implement due to the sealing requirements of the system and the difficulty in conveying a flow of cryogenic gas through a small-diameter tube over distances on the order of a meter or more. There is a need for an improved inverted dewar that circumvents these problems. The present invention fulfills this need, and further provides related advantages.